The present invention relates to a satellite-based vehicle communication/position determination system which performs two-way communication between a base station and a vehicle station using three or more geostationary satellites, and in which the base station performs position determination of the vehicle station.
Along with developments in satellite communication techniques, the advent of a satellite-based vehicle communication system is expected much. When a satellite-based vehicle communication system is constituted, one important issue is how to realize a communication/position determination function capable of determining the position of a vehicle station as well as a communication with the vehicle station by utilizing a wide areal property of the satellite communication. It is known that the position of a vehicle station can be accurately determined if at least three geostationary satellites are used.
More specifically, as shown in FIG. 1, a satellite-based vehicle communication/position determination system capable of achieving both communication and position determination functions is constituted by three geostationary satellites S.sub.1, S.sub.2, and S.sub.3, a vehicle station M having a low-directivity antenna 31 for performing radio-wave exchange with the geostationary satellites S.sub.1, S.sub.2, and S.sub.3, and a base station B having high-directivity antennas 32a, 32b, and 32c for independently performing radio-wave exchange with the three geostationary satellites S.sub.1, S.sub.2, and S.sub.3. A signal generated by the vehicle station M is transmitted to the base station B through three different transmission paths #1, #2, and #3. Note that communication need only utilize one of the three geostationary satellites, as a matter of course. As shown in FIG. 2, the base station B performing position determination detects transmission times t.sub.1, t.sub.2, and t.sub.3 in the three different transmission paths #1, #2, and #3, and calculates time differences t.sub.1 -t.sub.3 and t.sub.2 -t.sub.3 among the transmission paths. Thus, the position of the vehicle station M is determined in accordance with the intersections of curves based on these time differences.
A satellite-based vehicle communication/position determination system as described in an article entitled "Analysis of the GEOSTART Position Determination System" (L. O. Snively, W. P. Osborne: AIAA, 1986, pp. 50-58) is conventionally known. Although a detailed description is omitted, the conventional system utilizes a spread spectrum modulated signal which is expanded in all the bandwidth of a satellite network, as shown in FIG. 3. In this system, as shown in FIG. 4(A), a vehicle station called from the base station transmits a spread spectrum modulated signal which is diffusion-modulated by a high-speed pseudo random signal within a time interval of a predetermined frame period (T) designated by the base station. The base station receives vehicle signals via the three different transmission paths #1, #2, and #3 at separate antennas 32a, 32b, and 32c, and reproduces and outputs spread spectrum codes using a matching filter as shown in FIG. 4(B). The base station detects time differences among the reproduced spread spectrum codes, and determines the position of the vehicle station.
In the conventional system described above, since all the bandwidth of the satellite network is used, satellite power can be fully utilized. Since the high-speed spread spectrum modulated signal is received and processed by the matching filter, position determination can be performed with high precision. Thus, the conventional system can be an excellent position determination system. However, in view of communication, since a burst transmitting signal can only be transmitted, a communication mode is limited to a message communication mode, and a speech communication mode is impossible to realize. Since a spread spectrum system essentially requires high-speed modulation/demodulation operations, both the vehicle and base stations must have large-scale circuits.
Since an non-spread spectrum circuit normally utilizes a Delay Lock Loop, it takes much time to establish synchronization, resulting in a long processing time.